1. Field of the Invention
The present invention relates to a red luminescent compound and an organic electroluminescent device using the same, and more particularly, to a red luminescent compound and an organic electroluminescent device using the same that offers improved luminescent efficiency and driving voltage characteristics.
2. Description of the Related Art
Electroluminescent (EL) devices known as a self-luminous display have the advantages of large viewing angle, high contrast property, and short response time.
EL devices can be classified depending on the material composing their emissive layer into inorganic EL devices and organic EL devices. Organic EL devices have the advantages of higher luminance, lower driving voltage, shorter response time, and the ability to display a wider range of colors, over inorganic EL devices.
A general organic EL device includes an anode on the top surface of a substrate, with a hole transporting layer, an emissive layer, an electron transporting layer, and a cathode formed in sequence on the anode, wherein the hole transporting layer, the emissive layer, and the electron transporting layer are thin films made of organic compounds.
Organic EL devices having such a structure described above operate according to the following principles.
When a voltage is applied across the anode and the cathode, holes injected from the anode migrate via the hole transporting layer into the emissive layer. Electrons injected from the cathode migrate via the electron transporting layer into the emissive layer and combine with the holes therein to generate excitons. When the excitons transit from excited state to base state, molecules of the emissive layer emit light to form visible images.
Materials for the emissive layer can be classified depending on their light emission mechanism, into fluorescent materials that emit light from excitons in a singlet state and phosphorescent materials that emit light from excitons in a triplet state.
In general, phosphorescent materials are organometallic compounds containing a heavy atom and an organic ligand. In phosphorescence from phosphorescent materials, excitons in a non-emissive triplet state due to the heavy atom participate in emission via transition. Such phosphorescent materials use 75% of triplet state excitons for emission and offers higher luminescent efficiency compared to fluorescent materials using only 25% of singlet state excitons for emission.
Red luminescent materials, for example, DCJTB(4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran), have been developed. However, currently available red luminescent materials offer low luminescent efficiency and poor color purity.
Recently, research on phosphorescent materials including a second dopant such as rubrene or including an iridium metallic compound has conducted, as disclosed in U.S. 2002/0,121,638A1 and U.S. 2002/0,034,656 A1. However, these compounds still require improvements in luminescent efficiency and film stability.
Therefore, there is a need for the development of new red luminescent compounds applicable to red EL devices or full-color light emitting devices.